Plasma Sheath
A thin non-neutral positive boundary layer formed in a plasma adjacent to any physical wall or electrode.
Governing formula
V_sheath ≈ (k_B T_e / (2 e)) ln(m_i / (2 π m_e)), v_ion ≥ c_s = √(k_B T_e / m_i)
SI unit
V (Sheath Voltage) & m (Thickness ~ λ_D)
In depth
Because fast thermal electrons move faster than heavy ions, they strike boundary walls first, charging the wall negatively. This establishes a positive space-charge boundary sheath that repels electrons and accelerates ions toward the wall at or above ion sound speed (Bohm criterion).
Examples in the real world
Accelerating ions toward target substrate surfaces in semiconductor plasma etching reactors.